The purpose of this proposal is to evaluate the long term effects of acute venous thrombosis on the physiology of venous function and the development of the tissue changes commonly associated with the postphlebitic syndrome. The hypotheses to be tested include the following: (1) the syndrome of chronic venous insufficiency occurs only with involvement of the deep system; (2) the degree of disability can be related to the level and extent of the venous thrombosis; (3) patients with the most severe form will have the more proximal venous segments involved; (4) the development of the late stages can be predicted on the basis of the observed physiologic changes; and (5) preventative therapy can be used to prevent the irreversible tissue changes. The presence of acute venous thrombosis will be established by Doppler ultrasound, plethysmography and venography. The location and the effect of venous occlusion on maximal venous outflow, maximal venous reflux flow and valvular incompetence will be established by long term followup with repeat measurements.